Since the report in 1990 of electroluminescence (EL) in poly(.rho.-phenylene vinylene) (PPV) [1], EL of conjugated polymers has been considered an important property with many potential applications. Electroluminescence combined with other unique properties of polymers, such as solution processibility, band gap tunability, and mechanical flexibility, make conjugated polymers excellent candidates for low cost large area display applications. In addition to PPV, a variety of PPV derivatives and other conjugated polymers and copolymers have been found to exhibit electroluminescent properties [2,3]. Light-emitting devices incorporating these materials have demonstrated all the necessary colors needed for display applications.
Since the initial fabrication, a number of techniques have been developed to improve the device performance. One way is to use a low workfunction metal, such as Ca, as the electron injecting electrode (cathode) [4]. The double charge injection mechanism of polymer light-emitting diodes (LEDs) requires the match of cathode (anode) workfunction to the corresponding LUMO (HOMO) level of the polymer in order to achieve efficient charge injection. The relatively small electron affinity of most conjugated polymers requires metals with very low workfunctions to achieve efficient electron injection. However, since low workfunction metals are generally oxygen reactive, devices with low workfunction cathode are usually unstable. Thus, polymers with high electron affinities are desirable.
Another common technique is to incorporate charge transporting layers in a multilayer device structure. The charge transporting layer enhances the transport of one type of charge while blocking the other, achieving balanced charge injection and transport and spatially confined emission zone away from the electrodes. To date the highest efficiency polymer light-emitting devices reported are multilayer devices [5].
Pyridine-based conjugated polymers have been shown to be promising candidates for light-emitting devices [6,7]. As compared to phenylene-based analogues, one of the most important features of the pyridine based polymers is the higher electron affinity. As a consequence, the polymer is more resistant to oxidation and shows better electron transport properties. In contrast, most other conjugated polymers are susceptible to oxidation and exhibit better hole transport properties. FIG. 1 shows the structures of the pyridine-containing polymers and copolymers, namely poly(.rho.-pyridine) (PPy), poly(p-pyridyl vinylene) (PPyV), and copolymers of PPyV and PPV (PPyVP(R).sub.2 V) with various functional sidegroups R=C.sub.12 H.sub.25, OC.sub.16 H.sub.33, COOC.sub.12 H.sub.25. With respect to .pi. electronic levels, C.sub.12 H.sub.25 is slightly electron donating; OC.sub.16 H.sub.33 electron donating; and COOC.sub.12 H.sub.25 electron withdrawing. The pyridine-based polymers are highly luminescent, especially the copolymers. The internal photoluminescent quantum efficiencies of the copolymers have been measured [8] to be 75-90% in solution and 18-30% in film, with the exception of the OC.sub.16 H.sub.33 copolymer. The electron donating nature of OC.sub.16 H.sub.33 makes this copolymer more susceptible for oxidation. As a result, the PL quantum efficiency of the OC.sub.16 H.sub.33 copolymer is only 2% in film although it is high (.about.80%) in solution. To reduce the oxidation effects, the strapped copolymer (@PPyVPV) was introduced, as shown in FIG. 1(d). Also the strapped copolymer shows fewer aggregation effects as compared to the "usual" copolymers (see FIG. 1).
It is an object of the present invention to improve the performance of light-emitting polymers, such as reducing the required voltage required, and thus achieving similar levels of brightness while reducing the amount of power required for electroluminescence.
In view of the present disclosure or through practice of the present invention, other advantages may become apparent.